It is known that Internet Protocol (IP) is becoming one of the most widespread protocols for implementing transport layer in a communication network. In particular, Next Generation Networks (briefly, NGNs) are known, i.e. packet-based networks using IP at their transport layer. Such NGNs are able to make use of multiple broadband, QoS-enabled, transport technologies, while service-related functions are independent of the underlying transport layer technologies. It is expected that NGNs will enable delivery to users of enriched communication services, such as for instance VoIP (Voice Over Internet Protocol), video call, IPTV (Internet Protocol Television) and other multimedia communication services.
A communication network typically comprises a transport backbone and one or more access networks. While, in recent years, the transport backbone has experienced substantial growth, little has changed in the access networks. As a consequence, the “last mile” is still the main bottleneck between high-capacity Local Area Networks (LANs) and the transport backbone.
The most widespread solutions for implementing access networks today are Digital Subscriber Line (briefly, DSL) networks and Cable Modem (briefly, CM) networks. Although these solutions are an improvement compared to 56 Kbps dial-up lines, they are unable to provide enough bandwidth for the above mentioned enriched communication services.
More particularly, neither DSL nor cable modems can keep up with the ever growing bandwidth demand of such enriched communication services, since both technologies are built on top of existing communication infrastructures not optimised for data traffic. Indeed, in CM networks only a few channels are dedicated to transport of data, while the majority of bandwidth is used for transporting analog video signals. As to DSL networks, they do not allow sufficient data rates at required distances, due to signal distortion and crosstalk.
Therefore, most network operators have perceived that a data-centric solution optimised for providing IP-based enriched communication services can be necessary.
Passive Optical Networks (briefly, PONs) and, in particular, Gigabit Passive Optical Networks (briefly, GPONs) are currently considered among the best candidates for implementing access networks suitable for providing such IP-based enriched communication services. PONs are generally considered as an attractive solution to the “last mile” problem, since a PON minimizes the number of optical transceivers, central office terminations and fiber deployment.
A PON is a point-to-multipoint (P2MP) optical network with no active elements in the signals' path from source to destination. The only elements used in a PON are passive optical components, such as optical fiber, splices and splitters.
In particular, a PON typically comprises an optical line terminal (briefly termed OLT) and an optical distribution network (briefly termed ODN). The ODN comprises a plurality of passive optical components (typically spans of silica-based single-mode optical fibers and optical splitters) arranged so as to form a point-multipoint structure having a plurality of optical links radiating from the OLT.
The OLT is suitable for interfacing the ODN with a transport network, such as for instance a metropolitan area network (briefly, MAN) or a wide area network (briefly, WAN).
On the other hand, the ODN is suitable for allowing the OLT to exchange traffic with users connected at the far end of its optical links at transmission rates which can be higher than 100 Mbit/s. This advantageously allows the users to share the usage (and therefore the costs) of the OLT, thus allowing them to access broadband data services and broadband telephone services at acceptable costs.
When a PON is used for FTTB (Fiber To The Building) or FTTC (Fiber To The Curb) applications, each optical link of the ODN is terminated at its far end with a respective Optical Network Unit (briefly, ONU), which may be located either at the basement of a building or at the curb in the proximity of one or more buildings.
On the other hand, when the PON is used for FTTH (Fiber To The Home) applications, each optical link of the ODN has a plug at its far end, which is typically located within the user's home. A user wishing to access the broadband services supported by the PON may simply connect a suitable optical network termination (briefly termed ONT) directly to the plug located in his home, thus terminating the corresponding optical link.
In the present description and in the clams, the expression “optical termination device” will designate an optical device suitable for terminating an optical link of an ODN at its far end, i.e. either a ONU (in case of FTTB or FTTC applications) or a ONT (in case of FTTH applications).
Each time an optical termination device is connected to an optical link of an ODN and is switched on, the optical termination device typically has to be activated at the OLT. In particular, the ITU-T Recommendation G.984.3 (March 2008) discloses that the activation process is performed under the control of the OLT. The process is started by the OLT, which periodically checks for possible activation of new optical termination devices and/or possible reactivation of switched-off optical termination devices. According to the above mentioned ITU-T Recommendation G.984.3, the activation procedure includes three phases: Parameter Learning, Serial Number Acquisition, and Ranging.
During the Parameter Learning phase, the optical termination device, while remaining passive, acquires operating parameters to be used in the upstream transmission.
During the Serial Number Acquisition phase, the OLT discovers possible new optical termination devices by opening a window for upstream transmission (called “ranging window”) and asking for serial number transmission. In this ranging window, new optical termination devices send their respective serial numbers to the OLT. Upon reception of each serial number, the OLT associates it with an unused optical termination device identifier and sends it to the corresponding optical termination device.
The ITU-T Recommendation G.984.3 (March 2008), paragraph 7.2.2 discloses two methods for acquiring the serial number of an optical termination device.
According to a first method (“Configured-SN”), the serial number of the optical termination device is manually recorded at the OLT by the network provider before the optical termination device is switched on for the first time. Therefore, when the OLT detects the optical termination device, it retrieves from it its serial number and checks whether the retrieved serial number is one of the already stored ones. In the affirmative, the OLT activates the optical termination device, while in the negative the OLT stops the activation procedure.
On the other hand, according to the second method (“Discovered-SN”), the serial number of the ONT is automatically retrieved by the OLT when the optical termination device is detected for the first time. In particular, when the OLT detects the optical termination device, it retrieves from it its serial number and checks whether the retrieved serial number is one of the already stored ones. In the affirmative, the OLT activates the optical termination device, while in the negative the OLT determines that the optical termination device is being activated for the first time. In this latter case the OLT stores the retrieved serial number and activates the optical termination device.
During the Ranging phase, the OLT measures the optical distance between itself and each optical termination device, whose aim is described in detail herein after.
Transmission from optical termination devices to an OLT (which is also termed “uplink” or “upstream” transmission) is typically based on a Time Division Multiplexing technique. According to this technique, the OLT assigns to each optical termination device a respective “upstream frame”, i.e. a time frame during which the optical termination device is allowed to transmit upstream traffic to the OLT. Upstream frames of different optical termination devices are non-overlapping thereby allowing collisions of upstream traffic generated by different optical termination devices connected to the same OLT to be avoided.
Typically, during the above Ranging phase, the OLT retrieves from the optical termination device a set of physical parameters allowing the upstream frame to be assigned to the optical termination device to be determined. One of the most significant physical parameters for determining the upstream frame is the “round trip delay”. i.e. the time required for transmitting an optical signal from the OLT to the optical termination device and for receiving at the OLT a further optical signal in response to the optical signal from the optical termination device. For instance, the ITU-T Recommendation G.984.3 (March 3/2008), paragraph 10.4.3.3, discloses a procedure for measuring the round trip delay associated to an optical termination device, during the above Ranging phase.